Climate Change

Overview of climate impacts on the ocean

The warming of the global climate is unequivocal. The ocean plays a pivotal, if not underappreciated, role as a climate regulator. The ocean has absorbed the vast majority of global warming heat to date, roughly 93 percent of excess heat energy since the 1970s.¹ The ocean also captures about a quarter of anthropogenic carbon dioxide (CO₂) emissions emitted annually and will ultimately store the large majority of that carbon.²

While the marine environment provides an important buffer against the impacts of climate change to lessen atmospheric warming, this is creating a dangerous trio of impacts on the ocean: warming, acidification, and deoxygenation. Already, the cumulative effects of these impacts have contributed to profound and immediate changes around the world, from rising sea levels, increases in rainfall intensity, melting of ice in the polar regions, mass coral reef bleaching events, kelp die-offs, localized losses of shellfish due to acidic seawater, and shifting fish stocks.³ While extreme weather events are often seen as a more tangible manifestation of these changes, there are less visible shifts taking place below the surface of the water. The physical properties and chemistry of the ocean are changing at a faster rate than any other time in the past 300 million years.⁴

Indicators of a changing climate

Carbon dioxide emissions

Global CO₂ emissions have risen steadily in recent decades. Between 2008 and 2017, fossil fuel CO₂ emissions increased at a rate of 1.5 percent per year.⁵ As of early 2019, CO₂ emissions had reached 410 parts per million (ppm).⁶ Efforts to decarbonize have gained momentum in some parts of the world—notably through the increased use of renewables—but these efforts have not outpaced a growing global economy and the expanding use of coal, oil, and gas to meet global energy needs.

Global greenhouse gas (GHG) emissions currently show no signs of peaking.⁷ Current policies are expected to lead to warming of 3.3 degrees Celsius by 2100; under business-as-usual (a no-policy scenario), warming is projected to reach over 4 degrees Celsius by 2100.⁸ The Paris Agreement put forth a target of limiting global warming to 2 degrees Celsius compared to pre-industrial levels by 2100, and to “pursue efforts” to limit warming to 1.5 degrees Celsius. At current rates of warming, it is projected that the world will reach the 1.5-degree threshold between 2030 and 2052.

Ocean heat content

The year 2019 set a new record of ocean heating, surpassing 2018 as the warmest year on record for ocean heat content.⁹ The past five years (2015 to 2019) have ranked as the five warmest years for ocean heat content, confirming a trend that ocean warming has been accelerating since the 1990s.¹⁰ New research suggests that the rate of ocean warming is much faster than previously predicted. A series of analyses indicate that the ocean is absorbing excess heat 40 percent faster than the Intergovernmental Panel on Climate Change (IPCC) estimated in its Fifth Assessment Report in 2013.¹¹ This signifies that global warming may be more advanced than previously thought, potentially leaving the world less time to reduce emissions in order to meet global targets.

Ocean heat content is among the best—if not the best—indicator for climate change, given that the ocean stores the bulk of excess global warming heat.¹² For this reason, it is easier to use ocean heat content to detect the “fingerprint” of human influence on the climate, as opposed to the atmosphere which is more prone to year-to-year natural variability. The majority of heat (65 percent) has accumulated in the top 700 meters of the ocean; most of the remainder has accumulated in depths of 700 to 2000 meters.

Ocean acidification

As the ocean absorbs gigatons of the excess CO₂ that humans produce every year, seawater has become increasingly acidic. The rate of acidification is unparalleled in the past 66 million years.¹³ Changing ocean chemistry has substantial implications for marine food webs. For instance, ocean acidification, combined with rising temperatures, are making it more difficult for shellfish—including oysters, mussels, abalone, and pteropods—to form their shells. Shifts in the food web due to acidification and other climate stressors are already altering productivity and typical species interactions.¹⁴

Sea surface temperature

The average global sea surface temperature—the temperature from the upper few meters of the ocean—has risen about 0.12 degrees Celsius per decade over the past 50 years, with a higher rate of warming in recent years.¹⁵ These higher temperatures are contributing to: mass bleaching of coral reefs, die offs of other ecosystems with low thermal sensitivity (such as kelp and intertidal communities), shifts in fisheries stocks due to temperature, a reduction in algae productivity, and more frequent and intense extreme weather events.¹⁶

For coral reefs, for instance, increased sea temperature has contributed to more frequent bleaching events in recent years: globally, the annual risk of bleaching has increased from 8 percent in the early 1980s to 31 percent in 2016.¹⁷ The time between recurrent events has also become too short to allow sufficient time for recovery.¹⁸ Whereas the time between recurrent severe bleaching events was 27 years in the early 1980s, this window is now only 6 years.¹⁹ (Typically it takes 10 to 15 years for the fastest-growing corals to recover from a severe bleaching event.)

Mass coral bleaching has been one of the most tangible indicators of anthropogenic global warming, as coral reefs are particularly sensitive to thermal stress.²⁰ An estimated 30 to 60 percent of coral reefs have died since preindustrial times. Even if the goals of the Paris Climate Agreement are achieved (of limiting temperature to below 2 degrees Celsius), roughly 70 to 90 percent of current distribution of coral reef habitat will disappear by 2050.²¹ A failure to achieve the Paris Agreement will result in a near total loss of coral reefs by mid-century.²² These impacts will ripple through both natural and human communities: coral reefs serve as incubators for roughly 25 percent of all marine fish species, supporting fish stocks that provide food for more than one billion people.²³

Sea-level rise

The global sea level has been rising over the past century, though the rate of change has accelerated in recent decades. The rate of global sea-level rise nearly doubled from 1.7 mm annually throughout most of the twentieth century to 3.1 mm (0.12 inch) per year since 1993.²⁴ A global modeling analysis suggests that a 50-centimeter sea-level rise by 2100 would result in a loss of roughly 50 percent of coastal wetlands across the planet.²⁵ The impacts of sea-level rise holds consequence for both natural and human communities as structured coastal ecosystems (including coral reefs, saltmarshes, mangroves, and seagrasses) currently reduce wave height by an estimated 30-90 percent.²⁶ At least 40 percent of the global population lives in coastal areas which are vulnerable to rising seas.

About one-third of the rise in global sea level has been caused by the thermal expansion of water.²⁷ The increase in ocean heat in 2018 led to a 29.5 millimeter (1.2 inches) global mean sea-level rise above the 1981–2010 average.²⁸ Sea-level rise is further exacerbated by melting of ice sheets in the polar regions.

Ice melt and changes in the Arctic

Warming air and ocean temperatures are continuing to accelerate change across the polar regions. A recent study found that the Arctic surface temperatures have been warming at twice the rate as the rest of the planet for the past 50 years.²⁹ The Arctic has been warmer over the last five years than at any other time since records began in 1900.³⁰ This warming has led to dramatic reductions in sea ice thickness and extent, as well as accelerated melting of glaciers.

The 2018 Arctic Report Card, a peer-reviewed report that provides an annual status update on the region, found that the Arctic experienced the second-lowest overall sea-ice coverage ever recorded; the lowest-recorded winter ice in the Bering Sea; the second-warmest air temperatures; and earlier plankton blooms due to early melting of sea ice in the Bering Sea.³¹

Changes in the Arctic atmosphere and ocean—described by a NOAA scientist as the region’s “most unprecedented transition in history”³²—are a barometer for the rest of the planet. A recent study on ice loss in Antarctica found that the continent is contributing more to global sea level than previously estimated. Since 2012, the amount of ice loss per year has tripled, increasing the global sea level by 3 millimeters (0.12 inch).³³

By 2100, global sea level could rise 65 centimeters (21 inches) if the rate of sea-level rise continues to accelerate at the observed rate; ice melt from Antarctica could account for nearly 25 percent of this rise.³⁴ This scale of change would cause significant problems for coastal areas. In Brooklyn, for instance, which typically floods about once a year, this amount of sea-level rise would cause flooding about 20 times a year.³⁵

As Arctic waters warm and become increasingly ice-free, new implications are emerging for the rest of the world—from climatic and environmental impacts to geopolitical and commercial considerations. Growing scientific evidence has linked the loss of land and sea ice in the Arctic, and associated interference in the thermohaline circulation, to extreme weather events over the northern hemisphere—from severe winter storms in the eastern United States in 2018 to the intense cold wave in Europe in March 2018.³⁶

Some industries—such as shipping and oil and gas—see new opportunities for commercial expansion in the Arctic, with unknown consequences for the region’s fragile environment. In September 2018, the container ship Venta Maersk became the first container ship to traverse the Arctic Ocean via the Northern Sea Route. Meanwhile, scientists are working to better understand how climate change is upending the region’s food web and what it might mean for the future, from phytoplankton to polar bear populations.³⁷

At a macro level, there is a current lack of scientific understanding as to self-reinforcing feedback loops and the “known unknown” of tipping points, which represent critical thresholds beyond which the impacts of rapid climate change become difficult to reverse. A recent study suggested that Greenland’s massive ice sheet is melting four times faster than previously thought, and it may be approaching a dangerous tipping point, with serious implications for global sea-level rise within the next two decades.³⁸

Shifting distribution of marine species

As waters warm due to climate change, fish and other marine species are migrating into new territory, usually poleward, at a rate averaging 70 km (43 mi) per decade.³⁹ For example, in the U.S. Atlantic, three commercially important species—American lobster, red hake, and black seabass—moved northward by an average of 190 km (119 mi) between 1982 and 2015.⁴⁰ These shifts are expected to continue or accelerate in the future.⁴¹

Shifting stocks creates a challenge for fisheries governance given that current fisheries management lacks “future-proofing” policies to account for fluctuating or changing distributions of marine species. One study projected that new transboundary stocks will be present in roughly 30 percent of global EEZs by 2100.⁴² Stocks that straddle national boundaries may experience a scenario of “double jeopardy,” facing unsustainable competitive harvesting from multiple countries.

Emissions contribution of marine industries

Certain marine industries—including shipping, oil and gas, and fishing—carry a carbon footprint which has been growing in recent years but is rarely focused on in global climate negotiations and greenhouse gas assessments.⁴³ The shipping industry accounts for about 3 percent of global emissions; under a “business as usual (BAU)” scenario, it is projected that it could account for as much as a fifth of emissions by 2050.⁴⁴

Despite the shipping sector’s contribution to global emissions, it was originally excluded in the Paris Agreement in 2015. The International Maritime Organization announced in 2018 that the shipping industry would cut its emissions in half by 2050.⁴⁵ This level falls short of reaching Paris Agreement targets; however, certain industry leaders may pursue independent initiatives to accelerate the transition to commercially-viable carbon neutral vessels. In late 2018, for instance, Maersk announced a goal to reach carbon neutrality by 2050.⁴⁶

Emissions from the global fishing industry increased by 28 percent between 1990 and 2011⁴⁷ while average emissions per ton of landed seafood product grew by 21 percent during this time period.⁴⁸ Increased harvest from fuel-intensive crustacean fisheries primarily drove this growth in emissions. Capacity-enhancing fuel subsidies and growth in distant water fishing have also contributed to the fishing sector’s trend of increased emissions.

A 1.5-degree versus 2-degree Celsius future

Scientific consensus indicates that the ocean will experience significantly more serious impacts under even the most optimistic emissions reduction scenario.⁴⁹ In order to reach the Paris Agreement, GHG emissions in 2030 need to be approximately 25 percent lower than in 2017—which is technically still feasible—to put the world on a least-cost pathway to limiting global warming to 2 degrees Celsius (and 55 percent lower to limit warming to 1.5 degrees Celsius).⁵⁰

A major IPCC report examined the consequences of a future with warming of 1.5 or 2 degrees Celsius.⁵¹ While both scenarios would heavily impact the ocean, the report outlines that a half degree of warming will lead to vastly different futures for natural and human communities. By 2100, Arctic summers could be ice-free once a decade in a world with 2 degrees of warming, or once a century under 1.5 degrees of warming. Nearly all of coral reefs could be irreversibly destroyed under 2 degrees of warming, or 10 to 30 percent may persist if warming is limited to 1.5 degrees Celsius. Marine fisheries catch may decrease at an annual rate of more than 3 million tonnes in a two-degree world, versus about 1.5 million tones in world with 1.5 degrees Celsius of warming.

Future climate and ocean conditions will be determined by how aggressively humanity pursues climate adaptation and mitigation options. Limiting global warming would allow natural and human communities time to adapt and increase the chance of remaining below critical risk thresholds. Although current pledges to reduce global emissions fall short of what is needed to achieve the goals of the Paris Agreement, the next few years present a decisive window for decarbonization. For instance, each five-year delay in reaching peak emissions could lead to an additional 20 centimeter rise in sea level over the long term, presenting high adaptation costs for coastal cities.⁵² The technical capacity is available to dramatically reduce GHG emissions across all sectors of the global economy—energy, food and agriculture, buildings, and transport—though the transformation would require unprecedented speed and scale to curtail the worst impacts of warming.⁵³

Moreover, new research suggests that as compared to a high-emissions scenario of 3.5 degrees Celsius, achieving the Paris Agreement would result in significant benefits for ocean productivity, economies, and human communities. The study found that limiting warming to 1.5°C could increase global fishers’ revenues by USD 13.1 billion annually (due to higher fish biomass and ocean productivity), raise seafood workers’ income by USD 10.6 billion, and reduce household seafood expenditures by USD 18.3 billion.⁵⁴ The most significant gains would occur in developing countries, such as Kiribati, the Maldives, Indonesia, and other countries, which are most vulnerable to the impacts of warming temperatures and have the greatest reliance on fish for food security, incomes, and employment.